A longstanding problem in inverter design has been to supply alternating current (AC) to a load from a direct current (DC) source of electric energy while using electric switches which must have their conduction current interrupted in order to drive them into a nonconducting state. Often solid state switches such as thyristors and particularly silicon controlled rectifiers (SCR) are used as switches to switch the DC current into the conductors of the AC load. The switches must be turned on and off in time synchronization with the sinusoidal voltage variations required by an AC load.
A problem arises because of the internal characteristics of solid state switches such as many thyristors, particularly SCRs. SCRs can be turned on by application of a control pulse to a control terminal of the device, and when turned on such a device conducts current between its cathode and its anode. However, the device cannot be switched off except by interrupting the current flow. This switching characteristics of common solid state switches such as SCRs makes it difficult to design circuits to turn the switch off at a specified time. An approach to a solution to AC inverter design is shown in U.S. Pat. No. 4,353,023, issued to Kawada, et al, on Oct. 5, 1982, and is also shown in U.S. Pat. No. 4,434,393, issued to Kobari on Feb. 28, 1984, and also U.S. Pat. No. 4,426,610, issued to Kawada et al on Jan. 17, 1984.